Epoxy resin systems are used in the manufacturing of various articles, including composites. Examples of articles that are being evaluated for manufacturing from epoxy resin systems include windmill blades. Fabricating windmill blades includes a number of requirements for effective manufacturing especially when a resin infusion manufacturing process is used. One need is for reduced exothermic heat release during the epoxy resin system cure of the article (composite) in thicker sections of the article since in such sections, the exothermic heat released during cure cannot be easily conducted away from the article. If excessive temperatures are reached during the cure process, thermal degradation of the cured resin in the “hot spots” can occur with resultant mechanical property loss in the fabricated article.
Additionally, during cure, the article may undergo thermal shrinkage. Thermal shrinkage of a cured epoxy resin causes stresses to build up in a composite during cooling down from the maximum temperature reached at or after gelation. The stresses sometimes lead to interlaminar cracking in the article, with resultant loss of mechanical properties. The higher the temperature reached during cure after the gel point, the greater the amount of stress that will accumulate in the article during cooling.
Standard epoxy systems used for fabricating windmill blades are cured with stoichiometric quantities of aliphatic amines, usually primary amines. The systems generally have high cure exothermic temperatures, with the center of a 100-gram mass of resin/curing agent mixture often reaching a peak temperature of 250° C. or higher when cured in a 70° C. water bath, which water bath simulates typical mold conditions for windmill blade cure. Such cured articles frequently have indentations with areas of apparent “collapse” of the part due to thermal (and/or chemical) shrinkage.
Epoxy systems cured with anhydrides often have lower cure exothermic heat release than those cured with primary amines. However, anhydride-cured systems typically require higher mold temperatures than systems cured with primary aliphatic amines in order to reach an acceptable degree of cure and level of cured properties. Many fabricators of windmill blades lack the ability to heat the molds to the temperatures required for a typical anhydride cure.
Resin systems used for large commercial windmill blade fabrication normally must reach a cured glass transition temperature (Tg) of at least 70° C. in a mold itself held at 70° C. A fast development of glass transition temperature is highly desirable since the fast development enables the part to be removed from the mold sooner and thereby reduces mold cycle time, enabling more parts to be fabricated in one mold in a given amount of time.
Other requirements include the absence of highly volatile components in the system (for vacuum infusion and thermal cure). Systems for infusion applications require an initial mixed viscosity low enough (and rate of viscosity increase at the infusion temperature low enough) to enable the reinforcing fiber preform to be completely infused with resin before the resin system becomes too viscous for satisfactory flow through the fibers and fabric of the substrate. The requirement for low initial viscosity and long pot life becomes more stringent as the size of the windmill blade increases and hence, the distance the liquid resin must travel during infusion.
Epoxy resin systems for windmill blade fabrication must generally also meet certain cured mechanical property requirements such as a minimum tensile strength of ˜60 MPa, a minimum tensile modulus of ˜2500 MPa, and a minimum tensile elongation of ˜4%. Also, it is undesirable for the systems to contain components which are volatile enough that the system poses a combustibility hazard during normal fabrication conditions, or which are volatile enough that they tend to “boil” and form voids when the system is cured under vacuum.
In light of the above, there is a need in the art for curing agents for producing epoxy resin systems which have reduced exothermic heat release combined with desired cured mechanical properties when compared to the prior art resin compositions.